1. Field of the Invention
The present invention relates to a DC/DC converter.
2. Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., are each configured to operate receiving commercial AC electric power from an external circuit. Also, electronic devices such as laptop computers, cellular phone terminals, and PDAs (Personal Digital Assistants) are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (inverter) configured to perform AC/DC conversion of commercial AC voltage. Alternatively, such an inverter is configured as a built-in component included within an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing a basic configuration of an inverter. An inverter 1r mainly includes a fuse 2, an input capacitor Ci, a filter 4, a diode rectifier circuit 6, a smoothing capacitor Cs, and a DC/DC converter 10r. 
The commercial AC voltage VAC is input to the filter 4 via the fuse 2 and the input capacitor Ci. The filter 4 is configured to remove noise included in the commercial AC voltage VAC. The diode rectifier circuit 6 is configured as a diode bridge circuit configured to perform full-wave rectification of the commercial AC voltage VAC. The output voltage of the diode rectifier circuit 6 is smoothed by the smoothing capacitor Cs, thereby generating a converted DC voltage VIN.
An insulated DC/DC converter 10r is configured to receive the DC voltage VIN via an input terminal P1, to step down the DC voltage VIN thus received, and to supply an output voltage VOUT stabilized to the target value to a load (not shown) connected to an output terminal P2.
The DC/DC converter 10r includes a control circuit 100r, an output circuit 200, and a feedback circuit 210. The output circuit 200 includes a transformer T1, a first diode D1, a first output capacitor Co1, a switching transistor M1, and a detection resistor RS. The output circuit 200 has a typical topology, and accordingly, detailed description thereof will be omitted.
The switching transistor M1 is configured to perform switching so as to step down the input voltage VIN, thereby generating the output voltage VOUT. Furthermore, by adjusting the duty ratio of the switching performed by the switching transistor M1, the control circuit 100r is configured to stabilize the output voltage VOUT to a target value, and to control a coil current Ip that flows through a primary winding W1 of the transformer T1.
The detection resistor RS is arranged in series with the primary winding W1 of the transformer T1 and the switching transistor M1. A voltage drop (detection voltage) VS, which is proportional to the current Ip that flows through the primary winding W1 and the switching transistor M1, occurs across the detection resistor RS. The control circuit 100r is configured to control, based on the detection voltage VS, the current Ip that flows through the primary winding W1.
FIG. 2 is a circuit diagram showing a configuration of the DC/DC converter 10r investigated by the present inventors. The feedback circuit 210 is configured to generate a feedback voltage VFB that corresponds to the output voltage VOUT, and to supply the feedback voltage VFB thus generated to a feedback terminal (FB terminal) of the control circuit 100r. The feedback circuit 210 includes a shunt regulator 212 and a photocoupler 214. The shunt regulator 212 is configured to generate a feedback signal S11 having a level adjusted such that the difference between the output voltage VOUT and a predetermined target value becomes zero, and to supply the feedback signal S11 thus generated to a light-emitting diode of the photocoupler 214. A phototransistor (or otherwise a photodiode) of the photocoupler 214 is configured to convert a light signal S12 received from the light-emitting diode into the feedback voltage VFB that corresponds to the feedback signal S11.
On the primary winding side, the transformer T1 includes an auxiliary winding W3, in addition to the primary winding W1. The auxiliary winding W3, a second diode D2, and a second output capacitor Co2 form a second DC/DC converter. At the second output capacitor Co2, a DC voltage VCC develops according to the switching performed by the switching transistor M1. The DC voltage VCC is supplied to a power supply terminal VCC (VCC terminal) of the control circuit 100r. 
The control circuit 100r includes the switching transistor M1, a pulse modulator 102, a driver 104, and a current limiting circuit 120. The switching transistor M1 is arranged such that its drain is connected to a drain terminal DRAIN, and its source is connected to a detection terminal (SOURCE terminal). The DRAIN terminal is connected to the primary winding W1. The detection resistor RS is connected to the SOURCE terminal as an external component.
The pulse modulator 102 is configured to receive the feedback voltage VFB and the detection voltage V. The pulse modulator 102 is configured to generate a pulse signal SPWM having a duty ratio adjusted according to the feedback voltage VFB. The pulse modulator 102 is configured to control the timing at which the switching transistor M1 is turned off, according to the detection voltage VS which is proportional to the coil current Ip that flows through the switching transistor M1. Known examples of such a pulse modulator 102 includes an average current mode modulator, a peak current mode modulator, and so forth. The driver 104 is configured to instruct the switching transistor M1 to perform switching according to the pulse signal SPWM.
The current limiting circuit 120 is configured as a protection circuit configured to compare the detection voltage VS with a threshold voltage VCUR—LIM so as to detect an overcurrent state, and to suspend the switching performed by the switching transistor M1 if an overcurrent state is detected.
Related techniques are disclosed in Japanese Patent Application Laid Open No. H09-098571, and Japanese Patent Application Laid Open No. H02-211055.
For example, according to a set signal that is asserted with each predetermined cycle, the peak current mode pulse modulator 102 is configured to switch the pulse signal SPWM to a level (on level) that corresponds to the on state of the switching transistor M1. In a case in which the circuit operates normally, when the switching transistor M1 is turned on, the coil current Ip increases with a predetermined slope as time elapses. With such an arrangement, the detection voltage VS is compared with the feedback voltage VFB. When the detection voltage VS reaches the feedback voltage VFB after it rises, i.e., when the coil current Ip reaches the peak current level that corresponds to the feedback voltage VFB, the pulse modulator 102 is configured to switch the pulse signal SPWM to a level (off level) that corresponds to the off state of the switching transistor M1. When the next set signal is asserted, the pulse signal SPWM is again switched to the on level.
With the DC/DC converter 10r shown in FIG. 2, the detection resistor RS is connected to the control circuit 100r as an external component. Accordingly, in a case in which both ends of the detection resistor RS short-circuit due to contamination with dust or the like, the DC/DC converter 10r cannot detect the coil current Ip. Specifically, in such a case, the detection voltage VS is fixed to 0 V regardless of the value of the coil current Ip, leading to a problem in that the pulse signal SPWM is fixed to the on level. In this case, the current limiting circuit 120 cannot provide a circuit protection function. As a result, the switching transistor M1 repeatedly performs switching with a predetermined maximum duty ratio (e.g., 75%), which leads to a large amount of current flowing through the switching transistor M1 and the primary winding W1. Subsequently, the fuse 2 blows out and thus protects the circuit, or such a large amount of current has a negative influence on the reliability of the circuit before the fuse 2 blows out.
In order to solve such a problem, an arrangement is conceivable in which the detection resistor RS is built into the control circuit 100r. This is because such an arrangement in which the detection resistor RS is built into the control circuit 100r protects the detection resistor RS from short-circuiting due to dust or the like. However, such an arrangement in which the detection resistor RS is built into the control circuit 100r has a different problem in that the designer of the inverter 1r cannot change the electric power output from the DC/DC converter 10r. 